CALIFORNIA GULL usually attains a mostly to solidly medium gray back by early in its second winter. Thus, second-cycle California can suggest third-cycle plumages of large white-headed gulls and first-cycle plumages of small white-headed gulls. Note that it has a medium gray back, dark eyes, and often a gray-green hue to bill and tibia.

MEW GULL in first cycle can resemble second-cycle California (see above). Second-cycle Mew distinguished from third-cycle California by smaller size, smaller bill, and shorter wings, but plumage and bare parts similar: California more often has black bill ring and can have some orange-red at gonys; Mew usually has black subterminal marks on P4 (sometimes outer web of P3), Whereas P3-P4 typically lack black on California.

RING-BILLED GULL in first cycle can resemble second-cycle California (see above). Second-cycle Ring-billed distinguished from third-cycle California by smaller size and shorter wings; paler gray upperparts with poorly contrasting scapular and tertial crescents; poorly contrasting white trailing edge to upperwing; eyes often pale; and primaries lack distinct white tips at rest and distinct P9-P10 mirrors (often shown by third-cycle California).

AMERICAN HERRING GULL averages larger and bulkier with shorter wing projection, stouter bill; pink legs and bill base lack greenish or bluish hues, and bill rarely so neatly two-toned; eyes often pale; pale panel on upperwing more extensive (on inner 5-6 primaries versus usually 3-4 on California); gray back feathers paler (Kodak 4-5).

THAYER’S GULL typically lacks well-defined saddle, and gray of upperparts averages paler (Kodak 5-6), but some could closely resemble California in plumage: note Thayer’s shorter wing projection, less parallel-edged bill, and pink legs lacking blue-green hues. In flight, extensive pale inner primary panel and two-tone outer primaries of Thayer’s distinctive.

WESTERN GULL larger and bulkier with shorter wing projection, stouter bill bulbous tipped; pink legs lack the greenish or bluish cast often shown by California; bill rarely so neatly two-toned; and gray on upperparts darker (Kodak 8-11).

LESSER BLACK-BACKED GULL has extensively blackish bill in winter, more-variegated upperparts, and eyes often pale; gray in upperparts darker (Kodak 9-13); lacks strongly contrasting pale panel on inner primaries.

AMERICAN HERRING GULL and LESSER BLACK-BACKED GULL differ in much the same ways as do adults (see above and relevant accounts);
note California’s bill size and shape, medium gray upperparts, dark eyes, and gray-green to yellowish legs.